Thats because guns have line of sight, so if you rush with alot of melee, they are abit screwed, unless your against a few good players.
For your army tactics, you might want to start learning how to form and use multiple groups for your infantry and cav and learn how to preform surround attacks, hard flanking, rear attacks, sending your men in waves and not blobbing them up, positioning your guns to get flanking/back shots off enemy soldiered when your melee is in combat, and lastly don't bring bow infantry unless it is a daikyu sam, bows also suffer Line of sight so they kinda are whacky, plus they arent effective in multiplay.
if you have more questions, join us here and ask them.
http://steamcommunity.com/groups/THGWs